1. Field of the Invention
The present invention relates to an illumination device for illuminating a display area of a liquid crystal display device, and a liquid crystal display device using the same. Particularly, the invention relates to an illumination device which improves motion picture blur and a tailing phenomenon at the time of display of motion pictures, and a liquid crystal display device using the same.
2. Description of the Related Art
[First Related Art]
As alternate means of a CRT (Cathode Ray Tube) which is a conventional typical display device, in recent years, an active matrix type liquid crystal display device (hereinafter abbreviated to TFT-LCD) in which a TFT (Thin Film Transistor) or the like is provided as a switching element at each pixel has gone mainstream.
In principle, in the TFT-LCD, gradation data written into each pixel is held for one frame period (equal to a period of a vertical synchronization signal Vsync). In such a hold type display system, when a motion picture is displayed, the display can not respond to a quick image change, and degradation in picture quality can occur in which an image blur and a tailing phenomenon are visually recognized.
In order to solve this problem, a method is proposed in which a display period of gradation data of each pixel is limited to a specific period in one frame period in synchronization with a vertical synchronization signal Vsync (for example, see undermentioned patent document 1). Besides, in order to realize the method, a method is proposed in which an illumination area of an illumination device, such as a backlight unit, for illuminating an image display area of a TFT-LCD is divided into plural parts in the image display area, illumination of the respective divided areas is sequentially turned on and off, and a display period (illumination period) of each divided area is limited to a specific period in one frame period (for example, see patent undermentioned documents 2 to 5).
[Second Related Art]
More specifically, a cold cathode fluorescent lamp (CCFL) is used as a light source of a backlight unit for a conventional TFT-LCD, and a display area of the LCD is illuminated while the cold cathode fluorescent lamp is always turned on. When a motion picture display is carried out while the cold cathode fluorescent lamp is always in a lightening state, in the case where gradation data is rewritten in a frame period (period) of, for example, 16.7 ms and the motion picture is displayed, since a response time of a liquid crystal molecule to a change in electric field intensity is several tens ms, next gradation data is written before the response of the liquid crystal molecule is completed, and accordingly, there occurs a disadvantage that a “blur” is seen to be produced on the motion picture display.
Besides, in the TFT-LCD, since data written in a certain frame is held until the gradation data is rewritten in the next frame, a display blur called trail vision on the basis of a human engineer viewpoint is also visually recognized, and therefore, there is a problem that the degree of the blur of the motion picture becomes large.
The above problem is explained in detail in undermentioned nonpatent document 1 and nonpatent document 2. The nonpatent document 2 discloses a study to improve the motion picture blur by turning on and off the cold cathode fluorescent lamp of the backlight unit.
However, when the cold cathode fluorescent lamp of the backlight unit is simply turned on and off, an afterimage of a former frame remains, and this is visually recognized as a ghost of a moving body in an image. Especially in the case where a line segment is moved, a tailing phenomenon is visually recognized in which the line segment is seen doubly or triply, which causes the display quality to be remarkably degraded.
Then, as a countermeasure against the ghost, a scan backlight system is proposed in which a backlight unit is divided into plural areas and a light source of each divided area is turned on and off in synchronization with the writing of gradation data. In order to realize this, a direct type backlight unit is proposed in which plural light sources such as fluorescent lamps are arranged substantially in parallel to a gate bus line (scanning line), and the light sources are sequentially turned on and off for a plurality of the respective plural divided areas.
FIG. 74 shows a section obtained by cutting a direct type backlight unit, which is used for a conventional TFT-LCD to support a motion picture display, along a plane orthogonal to a tube axial direction of a cold cathode fluorescent lamp, and a brightness distribution of illumination light from the backlight unit. In FIG. 74, a gate bus line (not shown) of a TFT-LCD 1008 is extended in a direction vertical to a paper plane. Besides, a display start line of one frame exists at an “upper (top)” side of the left in the drawing, and a final display line exists at a “lower (bottom)” side of the right in the drawing. A backlight unit 1000 is divided into four areas from the “top” to the “bottom” of the drawing. The respective divided areas are separated by U-shaped lamp reflectors (reflection plate) 1002, and a cold cathode fluorescent lamp 1004 whose tube axis extends in the extending direction of the gate bus line is disposed in each of the lamp reflectors. A light emission port of the backlight unit 1000 is disposed at the rear surface of a display area of the TFT-LCD 1008 through a transmission diffused plate 1006.
[Third Related Art]
In recent years, the screen of the TFT-LCD 1008 has been enlarged and its brightness has been intensified, and also in the backlight unit 1000, there occurs a necessity to improve light emission brightness by increasing the number of luminous tubes.
Besides, as compared with a CRT, the TFT-LCD 1008 continues to output light for one frame, so that an image blur occurs in a motion picture display, and picture quality performance is inferior to the CRT of impulse light emission (undermentioned nonpatent document 3). In order to cope with this, the patent document 1 proposes a method of causing an LCD to perform an impulse operation, and a technique to realize an impulse operation is disclosed in undermentioned patent document 2 or patent document 6 in which the backlight unit 1000 is duty (flicker) driven in a unit of one frame, and in undermentioned patent document 7 in which image data and black writing are alternately performed. However, when the duty driving or black writing is merely performed, a light output time is reduced and the brightness of a display is lowered, and accordingly, it is necessary to raise the output of the backlight unit 1000 at the same time.
[Fourth Related Art]
Besides, in a scan type or a blinking type surface illumination device and liquid crystal display device, a cold cathode fluorescent lamp or an LED is used as a light source, and for the purpose of improving the quality of motion pictures (reducing the blur of a contour), duty driving is performed in which turning on and off a light is repeated at a frequency of 60 Hz.
[Fifth Related Art]
FIG. 75 shows a structure of a direct type backlight unit used for a conventional TFT-LCD to support a motion picture display when viewed from a display area side. As shown in FIG. 75, a backlight unit 1000 is divided into four areas from the top to the bottom of the drawing. Respective divided areas 1010 to 1013 are separated by lamp reflectors (reflection plates) 1002 (not shown in FIG. 75) having U-shaped sections. A cold cathode fluorescent lamp 1004 whose tube axis extends in the extending direction of a gate bus line of a TFT-LCD 1008 (not shown in FIG. 75) is disposed in each of the lamp reflectors 1002. A light emission port of the backlight unit 1000 is disposed at the rear surface of a display area of the TFT-LCD 1008 through a transmission diffused plate 1006. As a scan type illumination device, this direct type is mainstream.
FIG. 76 shows a structure of a sidelight type backlight unit as another scan type illumination device. As shown in FIG. 76, respective divided areas 1010 to 1013 of a backlight unit 1000 respectively include light guide plates 1020 optically separated from each other and arranged in a plane. A dot-like light source such as an LED 1022 is disposed at each of both end faces of each of the light guide plates 1020 to 1023.
Incidentally, the documents of the related art are as follows:
[Patent Document 1]
JP-A-9-325715
[Patent Document 2]
JP-A-11-202285
[Patent Document 3]
JP-A-11-202286
[Patent Document 4]
JP-A-2000-321551
[Patent Document 5]
JP-A-2001-125066
[Patent Document 6]
JP-A-5-303078
[Patent Document 7]
JP-A-2001-184034
[Patent Document 8]
JP-A-2000-194312
[Nonpatent Document 1]
Television Image Information Engineering Handbook, Ohmsha P70 to 71
[Nonpatent Document 2]
ASIA Display/IDW'01 P1779-1780, 1781-1782
[Nonpatent Document 3]
Yasuichiro Kurita, “Display System of Hold-Type Display and Picture Quality in Motion Picture Display”, Preprint of First LCD Forum
[Nonpatent Document 4]
J. Hirakata et al.: “High Quality TFT-LCD System for Moving Picture”, SID 2002 Digest, p. 1284-1287 (2002)
[Nonpatent Document 5]
D. Sasaki et al.: “Motion Picture Simulation for Designing High-Picture-Quality Hold-Type Displays”, SID 2002 Digest, p. 926-929 (2002)
[Nonpatent Document 6]
K. Sekiya et al.: “Eye-Trace Integration Effect on The Perception of Moving Pictures and A New Possibility for Reducing Blur on Hold-Type Displays”, SID 2002 Digest, p. 930-933 (2002)
[Nonpatent Document 7]
H. Ohtsuki et al.: “18. 1-inch XGA TFT-LCD with Wide Color Reproduction using High Power LED-Backlighting”, SID 2002 Digest, p. 1154-1157 (2002)
[Nonpatent Document 8]
Gerald Harbers, and two others, “LED Backlighting for LCD-HDTV, [online], Internet <URL: http://www.lumileds.com/pdfs/techpaperspres/IDMC_Paper.pdf>
[Problem of First Related Art]
In the case of the first related art, when the illumination light source is simply turned on and off, the display brightness is remarkably lowered, and there arises a problem that the LCD has low brightness and low picture quality. For example, in the case where the display area is divided into five divided areas, and illumination of 20% is sequentially performed in one frame, in the one frame period, the brightness becomes ⅕ as compared with the time of illumination of 100%. On the other hand, when a lighting time in each divided area is made long, although the brightness is raised, there arises a problem that degradation of picture quality such as motion blur becomes remarkable.
[Problem of Second Related Art]
In the direct type backlight unit 1000 of the second related art explained by using FIG. 74, since the cold cathode fluorescent lamp 1004 is disposed to be close to the rear surface of the TFT-LCD 1000, as shown in the upper stage of FIG. 74, there is a defect that uneven brightness is apt to occur. The horizontal axis of the upper stage of FIG. 74 indicates the position of the TFT-LCD 1008 on the rear surface of the display area, and the vertical axis indicates the brightness. In the direct type backlight unit 1000, as indicated by a brightness distribution curved line of the upper stage of FIG. 74, a difference in brightness is apt to occur between a place just above the cold cathode fluorescent lamp 1004 and a boundary of the adjacent cold cathode fluorescent lamps 1004, and there is a defect that uneven brightness is apt to occur by this. As a method of causing the difference in brightness to be inconspicuous, a method has been adopted in which a gap between the transmission diffused plate 1006 and the TFT-LCD 1008 is widened to diffuse and mix the illumination light, or a method has been adopted in which the degree of diffusion of the transmission diffused plate 1006 is raised to further diffuse and uniform the light emitted to a space just above the cold cathode fluorescent lamp 1004. However, the former has a problem that the thickness of the device is increased, and the latter has a problem that the diffused light is again incident on the cold cathode fluorescent lamp and is absorbed, and the light quantity is lowered.
[Problem of Third Related Art]
When the light emission brightness of the cold cathode fluorescent lamp 1004 of the backlight unit 1000 is raised to increase the brightness as in the third related art, there arises a problem that power consumption and cost are increased. Further, even in the case where an image having a low average brightness on a screen is displayed, the light emission brightness of the cold cathode fluorescent lamp 1004 remains high, and accordingly, the temperature of the TFT-LCD 1008 rises. It is also necessary to improve the cooling structure for suppressing this temperature rise, and according to circumstances, there arises a problem that the device volume of the TFT-LCD 1008 is increased.
[Problem of Fourth Related Art]
In the cold cathode fluorescent lamp or the LED, since current fed to cause light emission or power supply is restricted, there is a problem that the brightness can not be made high by the duty driving. That is, in order to increase the supplied current, a stabilizer of the cold cathode fluorescent lamp becomes large. Thus, the stabilizer becomes heavy and thick, and further its cost becomes high. Furthermore, there is a problem that with the increase of the current, the driving voltage becomes high, so that the current-to-light conversion efficiency of the cold cathode fluorescent lamp is lowered, and the lifetime becomes short. Besides, for example, in a display device of a portable electronic equipment such as a notebook computer, strict restrictions are imposed on the power supply. Also in a solid emission type light source such as an LED, there arises a problem that the current-to-light conversion efficiency is lowered by the current increase, and the lifetime becomes short.
[Problem of Fifth Related Art]
In the direct type backlight unit 1000 of the fifth related art described by use of FIG. 5, since the cold cathode fluorescent lamp 1004 is disposed to be close to the rear surface of the TFT-LCD 1008, there is a defect that the brightness distribution is apt to become irregular, and the uneven brightness on the display is apt to occur.
Besides, in the sidelight type backlight unit 1000 of the fourth related art described by use of FIG. 76, since a light source, such as the cold cathode fluorescent lamp 1004, having a relatively large light emission quantity and a long length can not be used, there is a problem that the brightness is low.